Return false when IndexError is raised

[sc68cal]

This fixes #286

[dlyssenko]

Hi @sc68cal, here's the problem description:

The issue has been introduced with PR#220:

• that PR introduced cached parsing of the running config, which is mostly indent-structured text
• however, we entirely overlooked a banner section

Unlike the rest of the config, the banner section(s) is a free text (i.e. unstructured) and therefore may produce:

• false positive results
• failures (like in your case)
• rogue / bogus / phony sections.

I have RC'ed your case - the failure occurs when a banner section contains a non-empty line containing only white spaces (in the rest of the config such lines are not allowed).

Your PR, indeed fixes that failure but does not solves the other issues, thus the code needs to be rewritten and UT fixture also needs to be updated respectively.

So, I'm going to close your PR and will file mine instead, where I take care of all the mentioned issues. The banner there will be parsed into a separate section.

Please test it as soon as I file it (later today / tomorrow). I'll update you.
thank you for catching this issue!

[dlyssenko]

The fixing PR is #291